Alex L. Dam
PHOTOGRAMMETRIC SCANNING INNOVATION IN THE LH SYSTEMS DSW500
Alex L. DAM, Scott B. MILLER, Gail A. NAGLE, A. Stewart WALKER
LH Systems, LLC, USA
dam@lh-systems.com, miller@lh-systems.com, nagle@lh-systems.com, walker@lh-systems.com
Working Group II/8
KEY WORDS: Scanner, Design, Hardware, Software, Photogrammetry, Data Processing, Radiometric Qualities.
ABSTRACT
The principal innovations in the DSW500 compared to the DSW300 include a stroboscopic light source, smaller
integrating sphere, lighter color wheel, and choices of computer platform (NT or Unix) and sensor (1.6 and 6.3
megapixel units as well as the 4.2). Roll film transport remains a popular option.
Extensive software improvements are being made to accommodate these innovations. New strategies for multiple
operating system optimization and code maintenance have been defined. Scan throughput and user interface evolve.
Work continues on the challenges of dynamic range, noise, color rendition and dodging. User friendliness has joined
high radiometric accuracy and geometric precision as a leading customer requirement. Great advances have been
occasioned by the replacement of the mechanically shuttered, continuous light source of previous DSW models with a
fully software controlled, flash illumination sequence generator. This unit’s wide dynamic range and greater
repeatability facilitate improvements in automation and radiometry. With the decreasing cost of computer memory,
radiometric correction algorithms that trade off memory for increased precision and advanced processing have appeared.
The overall goal is automatically optimized, high signal-to-noise imagery that can be tonally adjusted to operator
preferences.
Dodging of the frame artifacts due to sun angle and aerial camera lens vignetting has been enhanced. “On-line” dodging
performed on imagery acquired during a scan but before being saved to disk provides near real-time performance and
higher image quality.
1 INTRODUCTION
The DSW500 image scanner from LH Systems (figure 1) was introduced into the market place in summer 1999. The
DSW500 builds on the success of our previous model scanners including the DSW300, the DSW200, and the DSW 100
models. The DSW300 is described elsewhere (Dam and Walker, 1996) and the success of the design in terms of
geometric and radiometric performance has been confirmed by Baltsavias et al. (1998). As digital photogrammetry has
matured, and users have become more sophisticated, we have been keeping pace by updating and improving the
performance of our DSW product lines. The ever increasing demand for robust and automatic scanning operations has
Figure 1. DSW500 Digital Scanning Workstation on Windows NT® platform
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Alex L. Dam
driven the need and requirements for the DSW500. Table 1 shows the transition of the DSW product line and some of
the major changes.
Year introduced
1989
1995
1997
1999
Model
HAI-100/DSW
100
0.25 (x2)
DSW200
DSW300
DSW500
4.2
1.6, 4
1.6, 4.2, 6.3
12.5/25
5-15
4-20
4-20
8
10
10
10
Illumination
25W halogen
175W xenon
175W xenon
10W xenon strobe
Peak irradiance
(FC)
Host computer
options
300
3000
3000
30,000
MicroVAX/PC
25 MHz/16 MB
Sun Ultra
250 MHz/128 MB
Sun Ultra/PC
500 MHz/512 MB
Scanning time
(min.)
12.5 µm
b&w/color
Software version
60/180
Sun
SPARCstation
50 MHz/80 MB
10/30
5/14
3/7
1.0
2.0
3.0/4.0
4.2
Number of
configurations
Primary
function(s)
2
1
4
15
Triangulation
Scan cut film
Scan cut film
Scan roll film,
AIO Image
processing
New features
Image viewer
Scan Automation
Patch mosaic
Tonal curves,
Multi-format
Exposure setting
Roll film, projects,
color RadioCal
Roll film scanning
Scan roll film,
AIO, Image
processing
Dodging
Multi-platform,
rescaling, dodging
Image processing
CCD (million
pixels)
Pixel size range
(µm)
Bits
Table 1. Progress of the DSW scanners
2 ILLUMINATION
2.1 Xenon Strobe Light Source
One of the major improvements compared to the DSW300 is the change from a continuous xenon lamp to a xenon
strobe. The DSW scanners operate with area-based cameras that acquire images in a tile fashion across the photograph
to be scanned. The high precision (<2 µm RMS) stage permits butt matching of the image tiles into a continuous image
for the entire 10 x 10 inch stage area. By controlling the strobe pulses of light via software commands, the user has total
control over the amount of light reaching the CCD sensors. The quantity of light input, rather than the sensor integration
time, now determines image brightness. To appreciate the concept, it is useful to contrast it with the DSW300, which
uses the constant intensity, continuous output, illumination principle employed in almost all scanners today.
In the DSW300 light control, two methods were combined: a mechanical aperture for major light changes; and software
control of the shutter speed to vary sensor integration time. Varying integration time under constant illumination is
typically used in scanners in order to vary image brightness. Image brightness becomes proportional to CCD sensor
integration time or shutter speed. The DSW300 camera shutter usefully controls light over about an order a magnitude.
The “useful” aspect means before either shutter times become too short for good mechanical accuracy or too long for
good scanning speed. The mechanical aperture wheel could adjust the illumination over 6 factors of two, in order to
extend the useful shutter speed range of 10 to about 60. The operator would manually “dial in” the gross light setting
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based on film type and density, then finely adjust the image brightness using shutter speed. High lamp illumination
output was required since the maximum light had to be always available. In a “step and stare” scanning system like the
DSW has employed for over 10 years, illumination is only needed during camera integration, which is now about 25 to
50 percent of the scan time. The other 50 to 75 percent consists of stage movement from one capture center to the next,
frame transfer time and color wheel rotation time if color is selected. Even line scanners maintain illumination constantly
while not scanning, so in effect most scanners generally require high wattage, continuous output and waste lamp life.
Continuous arc type lighting requires warm up time to spectrally stabilize, so rapid cycling is not an option.
Owing to the “step and stare” approach, a major innovation leading to greater efficiency was feasible in the DSW500. A
low wattage, multiple pulse strobe illumination system was designed to be active only during the camera integration
time. The integration time itself does not affect the brightness of the image. Instead the amount of light energy generated
by the strobe is the determinant, while the purpose of the integration time is only to bracket the strobe pulses. Since the
stages are enclosed, the strobe will be the only significant illumination during frame capture, thus eliminating the need
for and limitations of the mechanical shutter. Because the flexibility of the strobe pulse generation is greater than a
mechanical shutter, the useful dynamic range controlled by the software is now based on it. This is much greater than the
old range and is also sufficient that the aperture wheel is no longer required. Thus we have eliminated slight problems
and unknowns due to mechanical control of shutter speed and aperture wheel. More importantly, we have added the
ability fully to control input film illumination, thus increasing the potential automation of the scan radiometry setup. For
example, during color balancing, regardless of film type or density, the software can automatically make a decision on
the optimum amount of light for each color band. Enhancements to the algorithms will meet customers’ expectations for
ease of use. The user may of course adjust light independently for each band during color scanning as well. This
flexibility permits the optimal sensitivity for each film type.
The technical design goals were based primarily on operator experience. They consisted of: factor of 60 dynamic range;
resolution of one part in a thousand over that range; 0.5% repeatability of illumination; automatic compensation for
lamp intensity falloff with aging; and easy lamp replacement by the operator. Since lamp life would now be based on
number of scans rather than number of power-up hours, the goal here was three years for single-shift operation, roughly
ten times longer than for halogen lamps, in order to give better overall cost/performance. These goals were achieved.
The lamp controller consists of a XILINX programmable gate array (PGA) driving a pulse trigger generator that fires
the strobe in conjunction with a 12-bit digital to analog converter (DAC) for changing the input reference voltage,
thereby changing the intensity of the pulse. These circuits were designed by Engineering Design Team (EDT) in
Beaverton, Oregon. They allow the number, frequency and intensity of the light pulses to be changeable by the host
software on a per-frame-capture basis, though they are typically constant for each band throughout a given scan.
Although it is possible, the operator does not normally control these three parameters directly: they are algorithmically
calculated from the operator’s single parameter image intensity control, for ease of use. This algorithm generates
parameter values based primarily on optimizing scanning speed. Currently, the strobe frequency can be set between 80
and 1000 Hz and the number, between 0 and 4000 pulses. Amplitude can span a range about a factor of 6 with one-part
in a 100 precision. This flexibility allows the software to choose the number of pulses desired in fine steps, providing
precise control over the amount of light.
2.2 Lighter Color Wheel
Color scanning on the DSW500 is achieved by keeping the stage plate stationary with respect to the sensor whilst capturing
the tile three times as a wheel rotates so that red, green and blue filters are placed in front of the light source. This is a
significant advantage, allowing independent light control per color band and there is precise registration of the color bands
since the stage and camera are stationary during these image captures. The weight of the color wheel is critical, because the
speed of rotation is a major factor in the total time taken for a color scan. Thus the unit in the DSW500 has been
substantially reduced in mass compared to that in the DSW300, resulting in faster rotation and much improved color scan
times, which are now less than 2.5x as long as for black and white.
3 WORKSTATION PLATFORM OPTIONS
With the increasing demand for NT based systems running on standard PC hardware, the DSW SCAN software has been
entirely ported to the NT operating system. Now, both Solaris based Sun systems and Windows NT based computers
can be used. Since the DSW is a high performance scanner, and thus places heavy demands on the PCI bus, we require
that only certain computer platforms can be used. This limits the amount of testing and robustness problems that can
occur. Today, we recommend Dell and Compaq workstation systems and Sun Ultra 10 or 60 workstations. These
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Alex L. Dam
computers are reliable and have been tested for the demanding operations of scanning, large data transfer rates, and
extensive on-the-fly computations such as image filtering, JPEG compression, resampling, etc. The speed of the DSW
continues to increase with increases in the clock speed of these systems. Table 2 below gives a summary of the host
computer platforms.
NT
NTworkstation
workstation
NT
NT4.0
4.0(service
(servicepack
pack44or
or5)
5)
Dell
DellPrecision
Precision410,
410,420,
420,or
or610
610workstation
workstationor
orCompaq
Compaq
Professional
ProfessionalWorkstation
WorkstationAP500
AP500
Dual
Dualprocessor
processorPII
PIIor
orPIII
PIII400
400MHz
MHzor
orgreater
greater
Built
Builtinin10/100
10/100Mb
Mbnetworking
networking
CD
CDROM
ROM
Minimum
Minimum512MB
512MBRAM
RAM
SCSI
SCSIdisks
diskspreferred
preferred
19
19inch
inchmonitor
monitorororgreater
greater(prefer
(preferTrinitron
Trinitrontype)
type)
24
24bit
bitgraphics
graphics
Consider
Consider8mm
8mmororDLT
DLTtape
tapedrive
drivesystems
systemsfor
forbackup
backupand
and
data
datatransfer
transfer
Consider
ConsiderCD
CDwriter
writer
Consider
Consideraalarge
largeamount
amountof
ofdisk
diskspace
spacefor
forautonomous
autonomous
roll
rollfilm
filmscanning
scanning
Recommend
RecommendPCI
PCIbus
busisisempty
emptyexcept
exceptfor
forDSW500
DSW500
camera
camerainterface
interfacecard
card
Sun
Sunworkstation
workstation
Solaris
Solaris2.6
2.6or
or 77
Ultra
Ultra10
10or
or60
60 workstation
workstation
333
333MHz
MHzor
or greater
greater with
with 2MB
2MB cache
cache
Built
Builtin
in10/100
10/100 Mb
Mb networking
networking
CD
CDROM
ROM
Minimum
Minimum256
256 MB
MB RAM
RAM (512
(512 MB
MB recommended
recommended
SCSI
SCSIdisks
diskspreferred
preferred
19
19inch
inchmonitor
monitor or
or greater
greater (prefer
(prefer Trinitron
Trinitron type)
type)
PGX
PGXgraphics
graphics or
or better
better
Consider
Consider8mm
8mmor
or DLT
DLT tape
tape for
for archive
archive
Consider
ConsiderCD
CDwriter
writer
Consider
Consideraalarge
large amount
amount of
of disk
disk space
space for
for autonomous
autonomous
roll
rollfilm
filmscanning
scanning
Table
Table2.2.DSW
DSWhost
hostcomputers
computers
4 SENSOR OPTIONS
With the introduction of the DSW500, three camera options for the customer are offered. The customer can now choose
between a range of scanning speeds and costs. These cameras are: Kodak 1.6i Megaplus – 1.6 million pixels; Kodak 4.2i
Megaplus – 4.2 million pixels; and Kodak 6.3i Megaplus – 6.3 million pixels.
Each of these cameras has 9 µm “full fill” CCD arrays. This means that the full 9 x 9 µm area of each pixel is light
sensitive. While the quality of each of these cameras is identical, the cost increases with the number of pixels in each
camera. The speed of scanning varies with each camera and with the density of the film and the number of bands to be
scanned. Table 3 lists typical scan speeds for each camera type under the NT operating system on a dual Pentium 500
MHz processor Dell computer. Times will vary slightly depending on CPU speed, disk types, film type, scanning area,
etc., and reductions continue to be achieved due to ongoing software improvements.
Camera Type
Black and white at 20 µm
Black and white at 14 µm
Black and white at 12.5 µm
Color at 20 µm
Color at 14 µm
Color at 12.5 µm
1.6i
3
5.5
7
8
15
18
4.2i
1.5
2.8
3.3
3
6.5
8
6.3i
1.5
2.1
2.6
3
5
6.5
Table 3. Typical scanning speeds for a single aerial photograph (in minutes)
The DSW500 with autonomous roll film feed generates huge volumes of data. As can be seen from the scan times
above, one scanner can produce a lot of scanned images in short order. Furthermore, very little overhead accrues if
options are turned on while scanning. These include JPEG compression, image sharpening, and image rescaling while
scanning on-the-fly. Thus a user can choose these options with very little penalty in scanning performance.
As with the DSW300, the physical scan pixel size is adjustable and can be physically set at any value between 4 and 20
µm. Once this is done, the user may choose to output the image at a pixel size that is a binary multiple of the physical
setting. In addition the image rescaling option enables absolutely any pixel size to be achieved by software resampling.
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5 SOFTWARE IMPROVEMENTS
5.1 Update of Tonal Transfer Curve
The DSW500 software offers the user a complete set of radiometric adjustment tools that greatly simplifies the process
of obtaining realistic color imagery with a full histogram. The tools are available for use with both the real-time image
browser and the pre-scan image in a fully integrated tonal package. These tools include the ability to preset the
radiometric parameters during the setup of a roll film or cut film job. This allows the user to pre-store the setting for
changes occurring over the project. These settings include the amount of light and the tonal transfer curve settings. The
tonal transfer “curve” is the method by which the radiometry can be adjusted in a very non-linear way. The setting of the
tonal transfer curve is a key to obtaining optimum scans from a variety of films and exposure types. These tools include
histogram normalization and colormap adjustments. The colormap adjustment tools have been greatly expanded. New
features include “undo” operations and histogram updates that reflect colormap changes. The time required to set up a
scan has been greatly reduced by internally storing ten bits of radiometric information during the pre-scan or during
browsing. This 10-bit radiometric data is then re-used for all tonal adjustments to obtain the optimum tonal transfer
curve. This technique allows the user to work with the full dynamic range of the sensor while making radiometric
adjustments even though the final output may be compressed to eight bits. Tonal controls now include the option of
superimposing the tonal characteristics of a different monitor during scan setup.
5.2 Calibration
Like the DSW300, the DSW500 is very stable and does not change significantly over time. However, regular
calibrations, about once a month, are recommended to keep users familiar with the operations and provide a history over
which any change can be understood. Both scanner models have stage and sensor calibrations, which together take about
an hour to complete after which operations can continue as normal. The geometric calibrations are identical and
automated.
Radiometric calibration on the DSW500 does the normal sensor flat-fielding for each individual color band on a pixel
by pixel basis. In addition, camera dark level is set for no light response. Camera gain, pulse frequency and pulse
amplitude are all set to achieve a standard average white point response when no film is being imaged. The DSW500
also offers a much more fully automated radiometric calibration since there is no manually controlled aperture setting
involved. The calibration adjusts the system to produce a constant illumination over the life of the light source resulting
in light settings that can be referenced to absolute film density.
6 CONCLUSIONS
The improvements in the DSW500 compared to the DSW300 are incremental rather than radical. The use of leading
edge technology in the form of a stroboscopic light source is exciting, but for the user is merely a means to higher
reliability, performance, and lower maintenance and parts replacement costs. The other developments draw less on
innovative hardware but are all the more critical for users. The choice of sensors results in three options at different
price points, giving users a choice. The characteristics of the 1.6i and 6.3i sensors are almost the same as the 4.2i except
for the numbers of pixels, so the challenge of introducing them has been predominantly a software one, plus one
hardware improvement – a smaller integrating sphere with a larger aperture in order to illuminate the whole field of view
of the 4.2i at all resolutions and of the 6.3i up to 15 µm. One might expect that many users would opt for the 1.6i to
minimize cost, but the 6.3i has proved remarkably popular, comprising around half of the units ordered so far. The
option of the 6.3i was added with two groups of customers in mind: scanning bureau service companies, for which speed
is of the essence; and specialist organizations from the intelligence world for which fast scanning at high resolutions
(<10 µm) is critical for normal operation. But it seems that speed is still attractive to a large proportion of customers.
Whether speed is simply exciting per se or whether users perceive the “delay” between the beginning of scanning a large
project and the beginning of automated triangulation as unacceptable, is not clear.
The addition of the Windows NT platform is the most obvious response to user needs. Many customers have standardized
on the NT platform. Given that the PCI-bus frame grabber card fits into both PCs and the latest Sun Ultras, and given that
porting to the new platform is greatly facilitated by modern software development tools, this step has been a straightforward
one, albeit very meticulously performed. Finally, however, the ongoing development of software is the real challenge that
underlines the very real difficulty of meeting customers’ requirements for superb radiometry and aesthetic appeal on all
kinds of imagery.
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The “step and stare”, modular design of the DSW500 leaves open the way to adopting innovations in illumination and
sensor technology as they appear. More efficient light sources in the future may well emerge that are more cost effective
and will yield more light without requiring more power. New sensor technology is rapidly improving. Since 1994, the
DSW has exclusively used the 10-bit Kodak Megaplus camera models, which, except for an increasing range of array
sizes, have not changed fundamentally. In 1999 Kodak sold its Megaplus subsidiary to Roper Scientific. So far this has
caused only a change in the logo placed on the cameras. However, basic improvements for some of these models are
imminent. Specifically, the 1.6i camera model is now being tested with Kodak’s new blue sensor chip. This CCD, which
uses a modified front-illuminated design, gives the camera greater overall sensitivity than the standard 1.6i CCD by a
factor of two. The key design change consists of semi-transparent electrodes that are open to much greater incoming
illumination. This is particularly true in the shorter blue wavelengths (hence the nickname), where up to four times the
previous quantum efficiency has been achieved owing to significantly less scattering than before. The prediction is that
the enhanced blue response should help scanners by improving sensitivity to the cyan-magenta color film dye layers.
This new 1.6i model is being evaluated by LH Systtems and should soon enter the DSW product line. Elimination of the
mechanical shutter requirement in the DSW500 has made it faster and easier to try new sensor and camera products. The
practical results of this innovation are expected to include not only improved radiometric quality but also greater speed.
Further radiometric advantages are likely to accrue a little further into the future with enhancements to the sensors
providing 12-bit rather than 10-bit data. Finally, sensors with larger CCD arrays are certain to appear in the marketplace at reasonable prices and these can be incorporated if their performance is interesting.
Minor mechanical improvements, that seem inevitably to be incorporated during the process of hand-building small
numbers of units every month for long periods, will lead to higher speed, as will the certain increases in computer speed,
in terms not only of processor performance but also bus and disk speeds. Progress will be made with dodging, balancing
and the other radiometric challenges. We must still work to make the setup of high quality scanning even easier for
inexperienced operators. And work in progress includes a solution for the familiar problem of scanning color negatives
in an optimal way tonally while still capturing the fiducial marks in a faithful manner.
REFERENCES
Baltsavias, E.P., Haering, S., Kersten, T., Dam, A., 1998. Geometric and radiometric evaluation of the DSW300 roll
film scanner. ISPRS Journal of Photogrammetry & Remote Sensing, Vol. 53, Part 4, pp. 217-234.
Dam, A., Walker, A.S., 1996. Recent developments in digital photogrammetric systems from Leica-Helava.
International Archives of Photogrammetry and Remote Sensing, Vol. 31, Part B2, Vienna, Austria, pp. 66-71.
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